1. Technical Field
This invention relates to multilayer sliding bearings of the type having a metal backing, a liner of bearing metal and a relatively soft metallic overlay.
2. Related Prior Art
Multilayer sliding bearings are typically used in high-load engine applications for journaling the crankshaft, connecting rods, and the like and include a rigid metal backing of steel on which a lining of bearing metal of either copper-lead or aluminum alloy is applied, over which a running layer of a relatively softer metal is overplated usually in the form of a single layer of lead-tin-copper alloy having a thickness of about 25 .mu.m. A nickel diffusion barrier or copper bonding layer is often interposed between the lining and overplate to prevent the tin of the overplate from diffusing into the bearing liner. As a final step, the bearing is typically coated with a micro-thin layer of tin or lead-tin flash plating, giving the bearing a bright, aesthetically pleasing appearance and providing a level of corrosion protection.
The flash plating present on the concave running surface of the bearing is quickly dissipated during the initial engine break-in period.
In service, such multilayer sliding bearings are subjected to high dynamic loading that varies in magnitude and direction due to the inertial loads applied by the piston and connecting rod mechanism and by the cylinder gas. The soft overplate layer enables the bearing surface to continually change and conform under high load forces to any misalignments or changes in profile or loading of the member being journaled, so that the loads are distributed across a greater surface area of the bearing. In addition to its conformability, the overplate also has the characteristic of embedibilty, which enables the overplate to embed any foreign hard particles of dirt and/or metal that may come between the bearing surface and the member being journaled to protect the bearing and member from excessive wear or damage.
It is generally accepted that conformability and embedibilty characteristics are dependent on overplate thickness, with a thicker overplate being preferred (i.e., on the order of 25 .mu.m or more). It is also generally known that as the thickness of the overplate increases, so does the susceptibility of the overplate to fatigue in which the overplate surface fractures under load. Resistance to fatigue cracking requires that the bearing surface exhibit sufficient tensile strength to enable it to undergo minor configuration changes without cracking. Thus, it is necessary to balance the competing properties of conformability and embedibilty with fatigue resistance when designing an engine bearing, particularly one that is subjected to high dynamic loading.
For many high load engine applications, a 25 .mu.m overplate of lead-tin-copper has been found to exhibit good conformability and embedibilty while possessing good fatigue resistance. However, as the output and efficiency of engines continually increases, so does the dynamic loads placed on the crank shaft and connecting rod bearings, increasing the potential for bearing fatigue. While greater fatigue resistance can be achieved by simply decreasing the thickness of the conventional single layer lead-tin-copper overplate to load the 25 .mu.m thickness, it is at the cost of the conformability and embedibilty characteristics.
In addition to the demands on bearings due to increased loading, it is environmentally advantageous to reduce or eliminate when possible the use of heavy metals, and particularly lead in sliding bearings. U.S. Pat. No. 5,056,936 discloses a sliding bearing having a multilayer overplate construction that employs conventional lead-containing alloys, namely lead-tin-copper as one of the layer materials.
Thus, their exist they need in the industry for an improved multilayer sliding bearing that can operate under extreme high dynamic loading conditions while exhibiting good fatigue resistance as well as good conformability and embedibilty characteristics without the usage of lead-containing alloys in the overplate.